In recent years, various rotary electric machines have needed to reduce noise. In vehicles, particularly, an engine noise has been reduced. With regard to a vehicle alternator, however, blades of a cooling fan, which rotate at relatively high speed, cause unpleasant sound by colliding with air.
To reduce such fan noise caused by the vehicle alternator, JP-A-9-289756 (U.S. Pat. No. 5,763,968) proposes the combination of diagonal flow-type blades and centrifugal flow-type blades of a cooling fan. The numbers of the diagonal flow-type blades and the centrifugal flow-type blades are determined such that one of the numbers of the diagonal flow-type blades and the centrifugal flow-type blades is not divisible by the other and both of the numbers have no common divisor other than one. By using this cooling fan, noise with predetermined order of interference is suppressed.
However, for maintaining some volume of cooling air, the level of component of each order increases. Therefore, it is difficult to reduce total noise resulting from the creation of the cooling air.
As another measure to reduce the fan noise, it is conceived to reduce the diameter of the fan. In this case, however, the volume of the cooling air is reduced. Therefore, it is difficult to employ such fan to the vehicle alternator, which needs high power output.